Increased nutrient supply to the Southern Ocean during the Holocene and its implications for the pre-industrial atmospheric CO2 rise

Abstract

A rise in the atmospheric CO2 concentration of ~20 parts per million over the course of the Holocene has long been recognized as exceptional among interglacials and is in need of explanation. Previous hypotheses involved natural or anthropogenic changes in terrestrial biomass, carbonate compensation in response to deglacial outgassing of oceanic CO2, and enhanced shallow water carbonate deposition. Here, we compile new and previously published fossil-bound nitrogen isotope records from the Southern Ocean that indicate a rise in surface nitrate concentration through the Holocene. When coupled with increasing or constant export production, these data suggest an acceleration of nitrate supply to the Southern Ocean surface from underlying deep water. This change would have weakened the ocean’s biological pump that stores CO2 in the ocean interior, possibly explaining the Holocene atmospheric CO2 rise. Over the Holocene, the circum-North Atlantic region cooled, and the formation of North Atlantic Deep Water appears to have slowed. Thus, the ‘seesaw’ in deep ocean ventilation between the North Atlantic and the Southern Ocean that has been invoked for millennial-scale events, deglaciations and the last interglacial period may have also operated, albeit in a more gradual form, over the Holocene.

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Fig. 1: Sediment core and deep sea coral locations relative to austral summer surface nitrate concentrations and oceanic fronts.
Fig. 2: Holocene records of fossil-bound δ15N and biogenic opal flux from the Southern Ocean, compared with atmospheric CO2 and with climate- and circulation-related records from the Northern Hemisphere.

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Acknowledgements

This study was supported by Swiss NSF grant PBEZP2_145695 to A.S.S., US NSF grants 1401489 and 1234664 to D.M.S., Swiss NSF grant PZ00P2_142424 to A.M.-G., grants PP00P2-144811 and PP00P2_172915 to S.L.J., by the Deutsche Forschungsgemeinschaft through grant Li1815/4 to J.A.L., by funding from the Swedish Research Council VR-349-2012-6278 to E.M., from the Natural Environment Research Council NE/N003861/1 to L.F.R., and from the French INSU/LEFE Indien Sud to A.M. This research was also supported by ExxonMobil through the Andlinger Center for Energy and the Environment at Princeton University and by the Grand Challenges Program of Princeton University. Cores MD11-3353 and MD12-3396CQ were retrieved during Indien Sud oceanographic cruises (A.M.) and we express our thanks to the crew of the R/V Marion Dufresne as well as the French Polar Institute (IPEV). The authors thank K. Hendry, M. Palmer and B. Heinz for their valuable input, and X. Crosta for his help with diatom species identification.

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A.S.S., D.M.S., A.M.-G. and G.H.H. designed the study. A.S.S. performed the δ15Ndb analyses and wrote the first draft of the manuscript with D.M.S., A.M.-G. and G.H.H. L.M.T., S.L.J. and J.A.L. contributed the 230Th-normalized opal flux data. E.M. and A.M. provided access to the sediment cores and measured the radiocarbon ages for the construction of the age model. L.F.R. and J.F.A. recovered the corals, and X.T.W. generated the coral-bound δ15N data. All authors contributed to the interpretation of the data and provided input to the final manuscript.

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Correspondence to Anja S. Studer.

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Studer, A.S., Sigman, D.M., Martínez-García, A. et al. Increased nutrient supply to the Southern Ocean during the Holocene and its implications for the pre-industrial atmospheric CO2 rise. Nature Geosci 11, 756–760 (2018). https://doi.org/10.1038/s41561-018-0191-8

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